Here's a look at what I think is a particularly good Lego molded dragon, the Hungarian Horntail from the Harry Potter set 4767 Harry and the Hungarian Horntail(ad link).
This species of dragon has appeared in several sets now, but other versions are brick built, while this one is made up from just a few specialist molded parts. While I of love building an animal, I have a real soft spot for pre-formed Lego animals, and as dragons go this one is especially satisfying, as despite single large part making up the body, it's still quite posable, and really fun to swoosh!